Since polyethylene-2,6-naphthalene dicarboxylate (simply abbreviated as PEN hereinafter) is superior to polyethylene terephthalate (simply abbreviated as PET hereinafter) in basic physical properties such as heat resistance, gas barrier properties and chemical resistance, it is expected to be useful as packaging materials for bottles (containers), sheet materials or the like. Therefore, there have been made a large number of proposals for the use of a blend of PEN and PET, a copolymer comprising recurring units of PEN and recurring units of PET, and PEN alone. Of the packaging materials, a material for use in drinking bottles for juice and the like is strongly desired to have excellent color and transparency, from the viewpoint of product value. Basically, PEN can be caused to react in the presence of the same catalyst system as that of PET, and the use of germanium dioxide as a polymerization catalyst is expected to be effective from the viewpoint of color.
JP-A 61-78828 discloses a process for producing a polyester through an ester interchange reaction and a subsequent polycondensation reaction, wherein the process is carried out in the presence of (a) a catalytic amount of an antimony compound, (b) a glycol-soluble manganese compound and/or magnesium compound, (c) an alkali metal compound and (d) at least one phosphorus compound selected from the group consisting of phosphorous acid, phosphoric acid and esters thereof, the amounts of the components (b) and (c) satisfying the following expressions (1) to (3): EQU M.ltoreq.50 (1) EQU A.ltoreq.60 (2) EQU A/M.gtoreq.0.73 (3)
wherein M is an amount of the component (b) used (mmol % based on the total amount of all acid components), and A is an amount of the component (c) used (mmol % based on the total amount of all acid components). PA1 the molar ratio of the terephthalic acid component to the 2,6-naphthalenedicarboxylic acid component is 80/20 to 95/5, and manganese, magnesium, phosphorus and optionally cobalt, are contained in such amounts that satisfy the following expressions (1) to (5): EQU 0.ltoreq.Mn.ltoreq.40 (1) EQU 0.ltoreq.Mg.ltoreq.80 (2) EQU 0&lt;Co.ltoreq.5 (3) EQU 15.ltoreq.Mn+1/2 Mg.ltoreq.40 (4) EQU 0.8.ltoreq.P/(Mn+Mg+Co).ltoreq.1.5 (5) PA1 wherein Mn is a proportion (mmol %) of manganese based on the total of all dicarboxylic acid components constituting the copolymer, Mg is a proportion (mmol %) of magnesium based on the total of all dicarboxylic acid components, Co is a proportion (mmol %) of cobalt based on the total of all dicarboxylic acid components, and P is a proportion (mmol %) of phosphorus based on the total of all dicarboxylic acid components.
However, this publication is silent about the use of a cobalt catalyst and has no disclosure of an ethylene naphthalate/ethylene-2,6-naphthalene dicarboxylate copolymer.
JP-A 7-223623 discloses a heat-resistant bottle that is made from a copolyester comprising 85 to 95 mol % of terephthalic acid and 5 to 15 mol % of 2,6-naphthalenedicarboxylic acid as acid components and ethylene glycol as an alcohol component, the copolyester having a glass transition temperature of 75.degree. C. or higher when it has a moisture content in an amount of 3,000 ppm or more. This publication, however, discloses only the use of calcium acetate as an ester-interchange catalyst. A polyester produced using an organic acid calcium such as calcium acetate as an ester-interchange catalyst is satisfactory from the viewpoint of the color of the polymer, but it has a tendency to produce large deposited particles in large quantities, thereby making it difficult to suppress whitening at the time of blow-molding a bottle. Since a copolyester having a large content of ethylene terephthalate has a higher crystallization speed than that of PEN on the whole, a catalyst system which can be generally used for the production of PEN for bottles cannot be used.
Furthermore, a polymer produced using a Ti-based catalyst as the catalyst system for suppressing deposited particles is deeply tinted with yellow and is therefore not suitable for use as a polymer for bottles. Although there is known a method for controlling this yellow tint by the addition of a cobalt-based catalyst, the polymer becomes blackish if the yellow tint is suppressed and hence, a co-PET having excellent performance cannot be obtained.